Eight UGA alleles have been obtained in S. cerevisiae. Two UGA alleles, ade5,7-143 and gal10-1, were isolated as forward mutations from wildtype. Six UGA alleles, arg4-2' (UGA), his5-2' (UGA), ilv3-1' (UGA), leu2'1' (UGA), lys1-1' (UGA), and asp5-1' (UGA), were derived from UAA alleles by selecting from revertants in the presence of UGA suppressors. With these 8 UGA alleles, two classes of UGA-specific suppressors can be distinguished. Suppressors in one class act upon all 8 allelqs ; while suppressors in the second class act upon all but 3 alleles : ade5,7-143, arg4-2' (UGA), and gal10-1.